A method for forming a photomask includes receiving a mask substrate including a protecting layer and a shielding layer formed thereon, removing portions of the shielding layer to form a patterned shielding layer, and providing a BSE detector to monitor the removing of the portions of the shielding layer. When a difference in BSE intensities obtained from the BSE detector is greater than approximately 30%, the removing of the portions of the shielding layer is stopped. The BSE intensity in following etching loops becomes stable.